Ring laser angular rate sensors, also called laser gyros, are well known in the art. One example of a ring laser angular rate sensor is U.S. Pat. No. 4,751,718 issued to Hanse, et al., which is incorporated herein by reference thereto. Present day ring laser angular rate sensors include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gap. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical closed-loop path.
The activation of the laser gyro's various subsystems at start-up may have ramifications for the life of the laser mirrors and other system components. A method is needed to orchestrate the various subsystems at start-up given each subsystem's start-up constraints.
Therefore, it is the motive of this invention to provide a modular laser gyro with a start-up method that provides a synchronized and effective start-up procedure that results in minimum delay and minimum adverse effects.
Laser gyros that utilize microprocessors for their control require that inertial navigation information, control information, test information, and status information be communicated to external systems including an inertial navigation system or a test system. The inclusion of a microprocessor in the laser gyro allows the implementation of new capabilities such as sending autonomous control functions and self testing along with self calibration and self diagnostics. This new capability requires the transmission and reception of a broad spectrum of data, some of which occurs at a high frequency rate.
Therefore it is another motive of this invention to provide a modular laser gyro with an improved communications and control method and apparatus.
Prior art high voltage power supplies for laser gyros used a 2,500 VDC large external power supply placed outside of the laser gyro housing. The external supply required high voltage feed-throughs into the laser gyro housing through a high voltage feed-through connector. The external high voltages also require special cabling and shielding: such high voltage feed-throughs are expensive. Such high voltage feed-through connectors are also difficult to construct while still maintaining a hermetically sealed housing for the laser gyro. Existing high voltage plastic seals may only maintain a vacuum to 10.sup.-6 Torr. In contrast, relatively inexpensive low voltage connector seals may handle a 10.sup.-9 Torr hermetic seal.
It is, therefore, another motive of the invention to provide a modular laser gyro incorporating voltage supply lines that can utilize an inexpensive, hermetic connector.
Associated with such sensors is an undesirable phenomenon called lock-in which has been recognized for some time in the prior art. In the prior art, the lock-in phenomenon has been addressed by rotationally oscillating or dithering such sensors. The rotational oscillation is typically provided by a dither motor. Dither motors of the prior art usually have a suspension system which includes, for example, an outer rim, a central hub member and a plurality of dither motor reeds which project radially from the hub member and are connected between the hub member and the rim. Conventionally, a set of piezoelectric elements serving as an actuator is connected to the suspension system. When actuated through the application of an electrical signal to the piezoelectric elements, the suspension system operates as a dither motor which causes the block of the sensor to oscillate angularly at the natural mechanical resonant frequency of the suspension system. This dither motion is superimposed upon the inertial rotation of the sensor in inertial space. Such dither motors may be used in connection with a single laser gyro, or to dither multiple laser gyros. The prior art includes various approaches to recover inertial rotation data free from dither effects.
It is, therefore, another motive of the invention to provide a modular laser gyro incorporating an improved dither drive and dither stripper which electrically removes (strips) this dither motion from the gyro output.
One technique for maintaining a constant path length is to detect the intensity of one or both of the laser beams and control the path length of the ring laser such that the intensity of one or both of the beams is at a maximum. U.S. Pat. No. 4,152,071 which issued May 1, 1979 to T. J. Podgorski, and is assigned to the assignee of the present invention, illustrates a control mechanism and circuitry as just described. Path length transducers for controlling the path length of the ring laser are well known, and particularly described in U.S. Pat. No. 3,581,227, which issued May 25, 1971 to T. J. Podgorski, U.S. Pat. No. 4,383,763, which issued May 17, 1983 to Hutchings et al and U.S. Pat. No. 4,267,478, which issued May 12, 1981 to Bo H. G. Ljung, et al. All these patents are incorporated herein by reference.
In the aforementioned patents, the beam intensity is either detected directly as illustrated in the aforementioned patents, or may be derived from what is referred to as the double beam signal such as that illustrated in U.S. Pat. No. 4,320,974, which issued on Mar. 23, 1982 to Bo H. G. Ljung, and is also incorporated herein by reference.
Herein "mode" is defined as the equivalent of one wavelength of the laser beam. For a helium-neon laser, one mode is equal to 0.6328 microns which is equal to 24.91 micro-inches.
In path length control systems of the prior art, the path length control finds mirror positioning for which the lasing polygon path length, i.e., the ring laser path length, is an integral number of wavelengths of the desired mode or frequency, as indicated by a spectral line, of the lasing gas. With proper design, the path length control forces the path length traversed by the laser beams to be a value which causes the laser beams to be at maximum power.
As is also known in the prior art, ring laser gyros are subject to small bias drift errors, and noise called random drift errors. Both of these errors may result in significant inaccuracies if the ring laser gyros are operated for extremely long periods of time.
Now referring to FIG. 56 which shows the results of experiments conducted by Honeywell Inc. of Minneapolis, Minn. which imply the existence of a ring laser gyro bias drift that is periodic. The typical bias magnitude change 20 C was on the order of (+/-) 0.01.degree./hr about a mean value shown as line 21A in FIG. 56. Bias magnitude changes, shown as curve 22B, were observed to be sinusoidal in nature with respect to mirror position shown as the X axis 19 in FIG. 56. The plot in FIG. 56 shows the bias magnitude change curve 22B in relation to the single beam signal curve 24B. The single beam signal curve 24B is derived from the magnitude of the AC component of the laser intensity monitor signal. Experimentally the bias was found to be 90.degree. out of phase, as shown by magnitude 26B, with the single beam signal curve 24B (SBS), but equal in period. Typically the average bias crossings 25 and 27 of the BIAS sinusoid curve 22B occur at the minimum or maximum of the SBS signal curve 24B.
The bias curve 22B is shown varying sinusoidally during one period of movement of the two mirrors 13 and 15. One period of movement is equivalent to two wavelengths. Even though the mirrors are moving, the system maintains a constant laser path 16 in the laser gyro 10, as shown in FIG. 1A.
The plot of FIG. 56 implies that as one mirror is moved "out" one wavelength and the other mirror is moved "in" one wavelength, for a total of two wavelength changes, the bias in the modular laser gyro 10 varies over one complete period. Ideally, the bias will vary uniformly as the mirrors are moved from an average bias point 25 to a negative maximum bias point 26B through an average bias again at point 27 to a maximum bias at point 28B to return to an average bias at point 5629. Those skilled in the art having the benefit of this new disclosure will recognize that with respect to the average bias 21A the integral of the bias curve 22B over one period of the curve from point 25 to point 5629 is zero, which implies that the total bias over the entire period is the average bias indicated by line 21A.
It is another motive of the present method and apparatus of the invention to exploit the above described phenomena to accomplish a bias drift improvement and random drift rate improvement.
It is highly desirable to know when the components of an inertial navigation system will fail. Life prediction is possible based on historic modular laser gyro performance data at particular temperatures. Lifetime prediction may be used to estimate when a device should be serviced for routine maintenance purposes. The ability to predict modular laser gyro lifetime allows modular laser gyro maintenance at highly desirable times such as nighttime or scheduled maintenance periods.
The capability of predicting lifetime is based on experimental and theoretical data showing that the output power of the modular laser gyro and a derived parameter, volts per mode is a function of both temperature and operating time. Typically, the longer a modular laser gyro is operational the lower the laser power output. Even though this power output diminishes slowly with time, after a considerable life the laser power output decreases below what is considered an acceptable level of laser power output. The acceptable level of laser power output is determined when the modular laser gyro is manufactured. Furthermore, it is also known that the power output of a modular laser gyro may fluctuate within a given temperature range. Therefore, it is desirable to look at a minimum power for a particular time of aging and a particular temperature range.
As a result it is another motivation of the invention to provide a highly reliable method of determining when a modular laser gyro may fail based on historic performance data for certain modular laser gyro performance parameters.
In operating a modular laser gyro it is important to maintain the laser beam current in each leg of the modular laser gyro between an anode and a cathode within a desired operating range such as, for example, about 0.15 ma to about 1.0 ma. In the prior art, large resistors called ballast resistors are employed to maintain stability of the plasma within the desired current range. Unfortunately, such ballast resistors tend to be very large resulting in a large amount of wasted power. Further, it is necessary to select these ballast resistors for each individual modular laser gyro out of a range of selectable ballast resistors. This selection or calibration of each modular laser gyro, results in higher production costs and less reliable current control than that which is provided by the present invention. Ballast resistors used in the prior art must be carefully selected in order to match the current in both legs to within better than one percent (1%) in order to reduce bias characteristics in the ring laser modular laser gyro. Further still, current control circuits of the prior art require high voltages and wide bandwidth circuits in order to achieve a high performance modular laser gyro.
It is another motive of the invention to overcome the deficiencies of the prior art by providing an active current control apparatus which does not require selected ballast resistors, uses conventional active elements and medium performance operational amplifiers, and yields a high performance modular laser gyro with no plasma oscillations over the entire operating range of desirable currents. Furthermore, through the use of a microprocessor based controller, the active current control apparatus of the invention maintains a high degree of accuracy and reliability in a modular laser gyro system application.
As a basis for designing the active current control apparatus of the present invention, design data was taken on a GG1320 model number modular laser gyro as manufactured by Honeywell Inc. of Minneapolis, Minn. The data taken was within the operating window of laser beam current with cathode current as a function of ballast resistor and with capacitance as a parameter. Since the 1320 model modular laser gyro operates in the negative resistance region of the current-voltage characteristic, stray capacitance near the anodes may significantly affect the operating window.
Operating windows as a function of current were obtained for the regions wherein plasma oscillations occurred. Ballast resistors as low as zero ohms and capacitance less than 15 pF had a very small effect on the operating window. This data was useful in defining the requirements for high voltage and low capacitance semiconductor devices employed in the instant invention.
Prior art modular laser gyro power supplies incorporated at least four large external power supply transformers. These transformers included a start transformer at 2,500 VDC, a run transformer at 750 VDC, a dither transformer and a PLC transformer at 330 VDC.
It is another motive of the invention to provide a modular laser gyro incorporating a single power supply transformer.
An integral part of a ring-modular laser gyro is the laser beam source or generator. One type of laser beam generator comprises electrodes and a discharge cavity in combination with a plurality of mirrors which define a closed path. The path is usually triangular but other paths such as rectangular may be used.
Present day ring-modular laser gyros employ a gas discharge cavity filled with a gas which is excited by an electric current passing between the electrodes ionizing the gas and creating a plasma. As is well understood by those skilled in the art, the ionized gas produces a population inversion which results in the emission of photons, and in the case of He--Ne, a visible light is generated which is indicative of the plasma. If the gas discharge cavity is properly positioned with respect to the plurality of mirrors, the excited gas may result in two counter-propagating laser beams traveling in opposite directions along an optical, closed-loop path defined by the mirrors.
In some embodiments of modular laser gyros, a unitary body provides the gas discharge cavity including the optical closed-loop path. Such a system is shown in U.S. Pat. No. 3,390,606 by Podgorski, which is assigned to the same assignee as the present invention. There an optical cavity is formed in a unitary block. A selected lasing gas is used to fill the optical cavity. Mirrors are positioned around the optical cavity at appropriate locations such that counter-propagating beams are reflected so as to travel in opposite directions along the optical cavity. A gas discharge is created in the gas filled optical cavity by means of an electrical current flowing in the gas between at least one anode and at least one cathode which are both in communication with the gas filled optical cavity.
It should be noted that prior art ring-modular laser gyro systems often have a pair of anodes and a single cathode which produce two electrical currents flowing in opposite directions. Each of the electrical discharge currents create plasma in the gas. Each current is established by an applied electrical potential, of sufficient magnitude, between one cathode and one anode. Alternately, the RLG may have two cathodes and one anode.
Various factors both external and internal to the RLG may effect beam intensity. Temperature is one external factor. A change in a cavity parameter is an example of an internal factor. In the prior art, RLGs are commonly operated with essentially a constant power or constant current input which results in a variable beam intensity due to external or internal factors. A certain magnitude of operating current is selected which under a specified range of external and internal conditions produces a beam whose intensity is adequate for satisfactory operation. However, it has been determined that the useful life of the cathode is a function of the magnitude, over time, of the current it must carry; the greater the magnitude the shorter the useful life of the cathode. In addition, the useful operating life of internal elements of the RLG, such as mirrors, is a function of the magnitude of the operating current; the higher the current, the shorter the operating life. These internal and external factors have caused RLGs to be operated with a higher current than necessary during part of their operating life in order to produce a beam intensity satisfactory for operation under all conditions, thus shortening the potential operational life of the RLG.
It is highly desirable for a modular laser gyro to be able to execute self tests in order to provide the inertial navigation system using the modular laser gyro to estimate its reliability and functionality. Therefore, it is another motive of this invention to provide a built in test capability into a modular laser gyro orchestrated by an integrated microcontroller.
In prior art designs, start up path length control was accomplished with the aid of a predetermined set point of the pick off voltage and the use of a voltage sweep. The desired set point was specified when the laser gyro was constructed. The laser gyros of the prior art had difficulty adjusting to two common effects, temperature fluctuations and fluctuations in system response due to aging. Therefore, it is a further motivation of the invention to provide a dynamic compensation mechanism capable of acquiring a particular laser mode, calculating volts per mode, and changing laser modes.